Aluminothermic welding is a process which has long been used to join together railway rails placed end to end.
For this purpose, a mould generally formed of three main parts is arranged around the ends of the rails to be welded, a crucible containing an aluminothermic charge is placed on said mould and said charge is ignited to initiate the aluminothermic reaction.
The molten metal derived from this reaction then flows into the mould and fills the moulding cavity.
After the metal has solidified and the mould has been released, the excess metal is removed and the weld region is polished to provide good continuity with the profile of the rails.
However, with this method the nature of weld metal is identical throughout the entire weld region, from the foot of the rail as far as the rail head.
Yet it is desirable, in some applications, to have a weld metal at the rail head which has different mechanical properties, and in particular which has greater hardness.
For example, for railway lines intended for transporting freight on which greater demand is placed, rails are used that are hardened at the rail head in order to extend their lifetime.
It is therefore desirable that the weld should have a hardness profile that is comparable with that of the rails i.e. greater hardness in the region of the rail head and greater ductility at the foot of the rail.
For this purpose, U.S. Pat. No. 6,227,282 has already described a method for aluminothermic welding in which an aluminothermic reaction is initiated in a crucible and the molten metal derived from this reaction is poured into the mould in which alloying additives containing a hardening agent have previously been placed via the diverting plug of the mould.
The additives may be in powder form contained in a capsule carried by the diverting plug or inserted in a cavity of the diverting plug.
The container containing the additives must be positioned in relation to the type of casting (uphill or downhill) so that only the metal intended to form the rail head region comes in contact with this head, and therefore the alloying of the aluminothermic metal with the additives containing the hardening agent only takes place in the region of the rail head.
However, while this method appears to be advantageous in theory, it does not in practice allow the expected localized hardening to be obtained.
The hardening agent is found to diffuse throughout the entire moulding cavity, which means that essentially homogeneous hardness is obtained over the full height of the rail.
It is therefore one objective of the present invention to provide an aluminothermic welding method which allows localized hardening of the weld at the rail head with good reliability and good reproducibility.
A further objective of the invention is to design a simple, robust device which can be used to implement this method.